1. Field of the Invention
This invention relates to hydrostatic power transmitting and speed reducing equipment having independent sumps which are useful in many diverse applications, one being for instance, a vehicle drive-line of the type generally known as a hydrostatic transaxle. This invention is particularly concerned with an improved transmission or transaxle having a housing with an interior space divided by a partitioning device into a first internal volume for the hydrostatic transmission mechanism and its associated operating fluid and a second internal volume accommodating a speed reduction mechanism in the form of a lubricated gear train.
2. Description of Related Art
Hydrostatic transaxles are increasingly being used in lawn care and other outdoor power equipment duties such as snow-blowing and have become the preferred choice for power transmission drive lines; for example, in lawn and garden tractors with most employing a single hydraulic pump fluidly connected to a single hydraulic motor. Although in most instances single motor hydrostatic transmissions are coupled by speed reduction gearing to a mechanical differential, applications also exist where two hydraulic motors are used and where each hydraulic motor is connected by a respective gear train to axle output shafts.
Furthermore, two hydraulic pumps can also be used with two such hydraulic motors to create a hydrostatic transmission for each drive wheel which can be useful for zero-turn radius vehicle applications. Occasionally, single motor hydrostatic transmissions are used without the addition of a mechanical differential, such that the hydraulic motor is coupled by speed reduction gearing to a single output shaft, and in these instances, the output shaft may be the axle driving one wheel of the vehicle or be arranged to drive the axle of the vehicle by an interconnecting chain drive.
All hydrostatic transmission require hydrostatic power transmission fluid in order to operate and the fluid acts as the medium to convey power between the pump and motor of the hydrostatic transmission. As the positive displacement fluid pumping mechanisms used by all hydrostatic transmissions and hydrostatic transaxles require careful and accurate manufacture to achieve the necessary close tolerance fits in order to minimize internal fluid leakage losses associated with high-pressure performance, a preferred practice is to prevent damaging contamination generated by general wear and tear in the power transmitting gear train from reaching the pressurized circuit of the hydrostatic transmission. By removing the chances for damaging particles of contamination from entering the hydrostatic pressurized circuit, especially important when sintered powder-metal gears are used in the gear train, a long and useful working life for the hydrostatic transmission can be expected.
Although by no means essential, it can nevertheless be desirable to position the hydrostatic mechanism in a fluid compartment which is physically separate from any adjacent compartments in which the gear train is located such that no exchange of fluid can take place and whereby damaging contamination in the gear train compartment remains confined to that compartment. Contamination containment by way of separate compartments is shown in U.S. Pat. No. 5,090,949 titled Variable Speed Transaxle, expressly incorporated herein by reference. Here a bulkhead is provided in the housing which carries a shaft seal, the shaft seal operating on the interconnecting drive shaft which mechanically couples the motor of the hydrostatic transmission in the hydrostatic compartment to the first reduction gear of the gear train in the adjacent gear train compartment. Further quantifiable benefits are gained as the compartment providing the sump for the gear train need only contain the bare minimum quantity of oil to satisfy lubrication considerations. Thus by relying on what in effect is xe2x80x9csplash lubricationxe2x80x9d, expense is saved as the quantity of fluid needed is less and the efficiency of power transmission is improved as the associated drag losses of the fluid contacting the rotating gears is much less then with a sump carrying a full capacity of oil.
On the other hand, with some hydrostatic transaxles, the hydrostatic transmission is arranged to operate within the same oil bath as the speed reduction gearing (and mechanical differential when included) and such designs are commonly referred to as xe2x80x9ccommon sumpxe2x80x9d types. Typically, the gear train and the hydrostatic transmission lie adjacent one another at the same elevation and the oil level in the sump is kept near to the brim to ensure that the hydrostatic components remain properly submerged at all times and also to avoid any ingestion of air. With a gear train operating submerged in the oil bath, power losses are greater due to the increase in fluid friction associated with the wetted area in contact with the oil than would be the case with the xe2x80x9csplash lubricationxe2x80x9d types mentioned earlier. Such gear drag losses can be especially noticeable in winter time when the gears are required to revolve from rest in a sump where the oil can be in an extremely viscous initial state, and the resulting higher than normal operational loads imposed on the components in the drive train are unavoidable. As it is not possible to select oils with different properties in the common sump design, a problem is posed as the optimum fluid type which would normally be selected as the preferred lubricant for a gearbox will have completely different characteristics as compared to the type of power transmission fluid most suited for the efficient operation of a hydrostatic transmission. Typically, a gear oil tends to be thicker with a high viscosity range whereas an automatic transmission fluid (xe2x80x9cATFxe2x80x9d) tends to be much thinner with a lower viscosity curve. As the hydrostatic transmission normally prevails when a conflict in design arises, it is accepted that the gear train may be operated in a generally adverse environment of low viscosity fluid such that accelerated wear and resulting higher contamination levels are more likely. The common sump design has the further limitation in that grease cannot be employed as the lubricant for the gear train. For certain applications, grease can be a more economic choice of lubricant.
Under normal atmospheric conditions, hydraulic fluids contain about 9% by volume of dissolved air which has virtually no effect on the physical properties of the fluid and therefore does not lead to any reduction in the performance of the system. However, should any appreciable quantity of undissolved air be present, the fluid will be prone to foaming problems, especially should the fluid experience excessive agitation, for instance, by any revolving elements such as gears being operated in only a partially submerged condition in the fluid sump. If such foaming occurs, it will rapidly lead to the destruction of the hydrostatic transmission.
It is also a physical characteristic of the fluid to expand and contract in volume in relation to changes in its temperature. In general terms, the volume of oil increases by about 0.7% for every increase in temperature of 10xc2x0 C., and as hydrostatic transaxles can operate at below sub-zero ambient temperatures as well as on occasion above 100xc2x0 C. oil temperature, it is necessary to include an additional dead space volume of about 8% to allow for such volume expansion over its initially contracted volume state. Accordingly, the fluid level in the sump rises and falls in relation to such temperature variation.
Quite often, an external expansion tank is fitted to the transaxle housing to allow for such expansion and contraction of the hydrostatic fluid. However, an external expansion tank can be troublesome as it is most often situated directly above the transaxle where little space exists. Frequently the space available under the frame of the vehicle is needed for rear-discharge ducts for the grass clippings. Therefore, there is often an advantage in casting the housing such that an additional space or void can be incorporated internally such that the need for an external expansion tank is avoided. Incorporating a breather vent in the housing directly above the void will allow the free flow of atmospheric air in either direction depending on temperature condition of the oil, and usually such a breather vent is positioned near or adjacent to where the largest gear resides, most often the ring gear of the differential. This works well so long as the air present in the void does not become too mixed up with the oil by rotating elements such as gears before the oil has sufficiently warmed to expel the air pocket from the void. Furthermore, as more oil has to be carried in a common sump transaxle as compared to a design having separate and distinct chambers for the hydro and gearing as mentioned earlier, a larger dead space volume has to be included to take care of the resulting increased volume expansion. Often, as the oil warms up towards its normal operating temperature, its expanded volume is not yet at a maximum, and, consequently, the remaining void or space situated in close proximity with the highest positioned gear still contains some air. This can cause considerable trouble as the gear, as its breaks through the surface of the oil, induces excessive agitation in the fluid, and the resulting mixture of air and oil in the sump can lead to foaming of the oil. Should such mixing occur to any great degree, this can be detrimental to the performance of the hydrostatic transmission as well as result in cavitation erosion on the load carrying bearing surfaces accompanied by pressure shocks and noises. The problem is further compounded should the undissolved air in the form of foam escape via the breather to pollute the environment.
A further problem can occur should the sump not be filled with the correct level of oil, as too low a level of oil can later cause the oil to aerate and foam when the transaxle is operated, whereas too much oil can result in it being expelled to the environment via the breather passage once it has expanded due to temperature rise. A typical problem encountered with vertical input shaft machines should the oil level be low is premature failure of the related bearing or seal due to a lack of lubrication. Furthermore, such naturally vented aspirated hydrostatic transaxles once left to cool after use in humid atmospheric conditions, draw moist air through the breather as soon as the oil begins to contract in volume and often this results in mist in the form of condensation of water vapor forming on the walls of the sump. Such entrained moisture, if not at once expelled as steam by the hot oil when the transaxle is once more in use, can even in small quantities over a period of time accelerate sludging of the oil by forming emulsions and by promoting the coagulation of insolubles such as dust particles that are also drawn through the breather as particles of solid matter as the unit cools after use. In general, air entering the sump causes the gradual oxidation of the oil and this deterioration in the lubricating properties of the oil ultimately lowers the life span of the hydrostatic transmission. Such a deterioration in the quality of the fluid can be rectified by oil changes at regular service intervals, but to undertake this is both costly and complicated to do, due to the nature of the construction of such transaxles.
Since the early 1960""s a number of solutions have been developed for the protection of fluid in a hydraulic reservoir from such problems associated with contaminated atmospheric environments. One such solution was a pliable device called the xe2x80x9cFawcett Breather Bag.xe2x80x9d The Fawcett breather bag, being a permanent flexible non-porous barrier, has the physical appearance of a synthetic rubber bag fully enclosed except for a metal stem giving access to the bag interior. As stated in its brochure, the Fawcett breather bag prevents atmospheric air and its associated contaminants from contacting the fluid in the reservoir. However, the Fawcett breather bag does not solve the problem of air trapped in the space between the bag and the fluid from getting mixed into the oil as undissolved air.
An alternative solution marketed by the Swiss company Angst+Pfister does however directly address this problem. Sales brochures of that product show an assortment of different breather bags designs, some of which have overcome the problem of air entrapment in the tank including one type shown formed in the shape of a bellows mounted externally to the top of the reservoir tank. A similar design of bellows is disclosed in U.S. Pat. No. 4,987,796 which is expressly incorporated by reference herein. This particular bellows differs in that it operates in an inverted sense and is mounted internally in the fluid reservoir to one side of the housing. With such a corrugated configuration exposed to the environment, it could be prone to clogging in dirty environments once there a sufficient accumulation of airborne debris settled at the bottom of the folds which would hamper and impede its natural free movement. However, neither bellows type or for that matter the Fawcett breather bag solves the practical problem should too much fluid be inadvertently poured into the reservoir such that the expansion volume is insufficient to allow for full fluid expansion. Once pliable devices such as these have deformed to their maximum extent, any further expansion of the fluid will cause the pressure in the reservoir to rise to the point where the fluid will leak at the point of least resistance. Such leakage, quite likely to occur at the interface between the pliable device and the housing, is polluting for the environment and would especially be a problem with the pliable device shown in U.S. Pat. No. 4,987,796 as its location is below the uppermost oil surface. Gradual leakage could furthermore take place should there be any manufacturing defects or imperfections on the surface to which the pliable device is engaged.
There therefore would be an advantage to be able to take care of volume changes in the hydrostatic compartment without recourse to using an external expansion tank or reliance on an externally vented bellows apparatus.
Hydrostatic transmissions also tend to be quieter in operation and work more efficiently and effectively when the fluid within the low-pressure side of the closed-loop circuit is charged or boosted from an auxiliary pump. The addition of such an auxiliary pump increases the manufacturing cost of a hydrostatic transmission and often requires a higher power output from the engine in order to drive both the auxiliary pump and the main pump of the hydrostatic transmission. There would therefore be a further advantage if the hydrostatic circuit could be pressurized without having to include an auxiliary pump.
It is one of the objects of this invention to create a positive head on the hydrostatic fluid entering the low-pressure passage of the hydrostatic transmission without recourse to using a charge pump. Preferably the compartment containing the gear train is sealed from the environment, and rising in pressure in the gear compartment aided or induced by the expanding volume of fluid in the hydrostatic compartment produces a net increase of pressure experienced by low-pressure passage of the hydrostatic transmission.
According to a preferred embodiment of the invention, the surface level of lubricant in the gear sump is automatically adjusted in direct proportion to the operational temperature of the fluid contained within the hydrostatic chamber. Having initially a low level of lubricant in the gear sump on the one hand lessens the adverse effect of power-retarding drag losses, especially during cold weather winter operation, whereas on the other hand, a rising level of lubricant in the gear sump can ensure good lubrication even when temperatures are elevated and viscosity is low. It is therefore a still further object of the invention to enhance the operational characteristics for the hydrostatic transmission by performance matching with respect to the operation of the speed reduction assembly irrespective of the temperature conditions in the environment.
In one form thereof, the invention is embodied as a hydrostatic and gear transmission having an integrated or combined housing formation whereby the interior space provided by the housing formation can be said to divided by a deformable non-permeable partitioning device into a region expressly used for the purpose of accommodating components comprising the hydrostatic transmission and a further region expressly used for the purpose of accommodating components of the gear transmission. The first region is completely filled with hydrostatic fluid and hermetically sealed from both the gear lubricant contained in the second region and the ambient air atmosphere environment surrounding the housing, and where any volume change in the fluid capacity of said first region due to temperature change is assimilated by the partitioning device to effect an equal but opposite volume change in said second region. The partitioning device should be pliable with the inherent characteristic of being easily elastically deformable to take up a change in the amount of hydrostatic transmission fluid in the first region, for instance, due to temperature changes of the fluid, and thereby facilitates the regulation in depth of lubricant held by the second region. Such elastic deformation of the partitioning device can occur for instance, as a result of an increase in fluid pressure above atmospheric pressure within the first region caused by the hydrostatic fluid expanding in volume and exerting a force on the partitioning device.
According to the invention in an another aspect, the housing may include an internal wall structure or bulkhead having an aperture positioned directly adjacent both the first and second regions. The partitioning device is arranged to reside juxtapose the aperture in a manner whereby to the one side of the partitioning device lies the region containing hydrostatic power transmission fluid and to the opposite side lies the lubricant for the speed reduction apparatus that may or may not also contain a mechanical differential. The hydrostatic region preferably 1 remains full to capacity at all times with power transmission fluid whereas the region containing the speed reducing device need only be with lubricant to a certain level that does not necessarily correspond with its full capacity. In the practical operating spectrum intended for the invention, the partitioning device preferably has an initial position set at about 0xc2x0 C., which corresponds to a contracted volume state of the hydrostatic fluid in the hydrostatic region and the lowest level of lubricant in the gear region, and a final position state set at about 110xc2x0 C., which corresponds to the maximum expanded volume state of the hydrostatic fluid in the hydrostatic region and the highest level of lubricant in the gear region.
It is a still further preferred feature of the invention to situate the partitioning device such that its expanding and contracting motion occurs substantially along a vertical axis with respect to the ground to cause a change in the level of lubricant in the gear sump, and for fluid on the one side of the partitioning device to be in effect counterbalanced by lubricant on the opposite side.
Filling the hydrostatic chamber with power transmission fluid can be time consuming at the factory, especially as there are often air pockets remaining which are difficult to remove without first operating the hydrostatic transmission. Such air pockets are normally not a problem for hydrostatic units fitted with breathers, as such trapped air can eventually escape. However, when a hydrostatic transmission has to operate without a breather, any such trapped air, if present, will remain incarcerated inside the hydrostatic chamber and is likely to cause poor operational performance and objectionable noise. What is therefore needed is a new solution that will not only ensure that air pockets are easily eliminated at the factory but also allow the fluid level to be easily re-checked in the field. According to a preferred embodiment of the invention, the partitioning device is fastened to the housing before the hydrostatic chamber is filled with fluid. It is therefore a preferred feature of the invention to provide a fluid filling plug on the exterior of the housing enabling the hydrostatic chamber to be exposed for fluid level inspection and for the partitioning device to be correctly positioned. Correct positioning of the partitioning device can be achieved by blowing compressed air through the hole for the bung into the gear chamber before the filling plug is fastened to the housing thereby closing off the hydrostatic chamber and thereby setting the position of the partitioning device. If necessary, once the hydrostatic machine has been factory tested to ensure it functions as intended, the screw plug on the housing which closes the hydrostatic chamber can be removed to allow any remaining air that may have floated to the surface to escape to atmosphere as well as allowing the topping-up of fluid if it should be required. Compressed air can again be blown into the gear chamber to correctly re-position the partitioning device before the filling plug is tightened to seal against the housing. Even so, should subsequent checks be necessary, the fluid level can be checked by any service agent who has the correct indication depth stick and access to a compressed air appliance.
Any noticeable leakage of lubricant to the environment is unacceptable and according to the preferred embodiment of the invention, any slight leakage of fluid from the hydrostatic chamber, for instance due to a manufacturing defect at interface between the housing and the partitioning device or initial overfilling of the chamber, can be captured internally. Therefore, according to the invention in another aspect, a leakage capturing system in the form of the gear train compartment is provided for the collection of unintentional discharges of fluid from the hydrostatic compartment.